This invention relates to the recovery of oil from subterranean formations.
In the drilling and production of fluid mineral deposits such as petroleum oil and gas from subterranean reservoirs, the presence of water (usually in the form of an aqueous brine solution) has been a continuing problem. In almost every subterranean formation wherein either petroleum oil or gas is present in quantities which make production practical, water is also found to be present. Usually, certain portions or strata of an oil-bearing formation may be considered oil-rich, and other portions or strata of the formation may be considered water-rich.
Production of connate water along with oil or gas from a producing formation is not desired for many reasons, among them being the extra production expenses encountered, the necessity for separation of the oil and water following production, and the fact that means must be provided for getting rid of undesired brackish water which is a pollutant to the surrounding terrain.
The problem is especially critical in those instances wherein a post-primary recovery process is employed which utilizes a liquid flooding medium such as water (i.e., a waterflood operation) that is injected into the formation, as through an injection well. By the very nature of such processes, a relatively large amount of injected water will be produced at the producing well or wells. The focus of the problem in such situations thus shifts to that of producing enough oil along with the water (connate and injected) to make the process economical, and the recovery of available oil from the formation.
The problem as set forth above and the many disadvantages encountered have been recognized by the oil and gas production industry for many years. Many efforts have been made to solve the problem in whole or in part to yield production at a more favorable oil-to-water production ratio and to provide for more complete recovery of available oil in subsequent waterflooding processes.
One prior art attempt at solution of the problem has been to use various solvents to selectively or preferentially plug water-rich portions.
Applicant himself has proposed various solutions. See, for example, Applicant's prior U.S. Pat. Nos. 3,749,174 issued July 31, 1973; 3,811,508 issued May 21, 1974; 3,865,189, issued Feb. 11, 1975; 3,866,684, issued Feb. 18, 1975; and 3,866,685 also issued Feb. 18, 1975.
Another approach taken by the prior art has been that of West, U.S. Pat. No. 3,181,612, wherein a polymer which swells on contact with water is injected into the well. There, however, a polyurethane polymer is utilized which swells only to about 10 times or so in volume compared to its dry volume. The polyurethane polymer of West is prepared in an oil-base solution for injection.
Many of the prior art methods have been at least partially successful, and have afforded at least partial solution to the problem in certain instances and under certain well conditions. Nevertheless, the problem mentioned above still plagues the industry and remains critical. Indeed, with the rapid increase in production costs and the enforcement of pollution regulations by various governmental agencies, the industry is in very great need for a practical and inexpensive solution to this long-felt problem.
One of the primary difficulties with the prior art solutions is their impracticality for use in most situations because of the expense involved. It must be remembered in connection with the methods of this type that, typically, huge volumes of materials must be injected into the formations. Since the oil or gas which is produced is of limited value to begin with, and since production expenses are already high, any material injected into the well for selective plugging or for any other reason must, to be at all practical, be quite inexpensive. Most of the methods of the prior art known to applicant entail the use of materials which are much too expensive to use in large volumes, in the typical situation encountered by the driller or operator.
Many other less serious problems have been found to exist with the various prior art proposals. For example, in some instances exceedingly large pump pressures have been found necessary to inject the solutions proposed. In other instances the solutions have gelled or set up prematurely before reaching the areas of primary concern thus leaving some areas completely unplugged. In still other instances it has proven impractical or too expensive to heat the well to the extent desired by some of the proposed solutions. In other cases, it has been found that the methods were not satisfactory for use under typical well conditions of salinity and/or temperature.
Further, operators are generally skeptical about injecting foreign matter which might have an adverse effect on the wells and even irreparably damage them.
It would be desirable if convenient and inexpensive methods for the recovery of oil utilizing selective plugging could be provided, wherein some of the more prevalent problems of the prior art could be eliminated.
The present invention provides such methods.